Synthetic biologists have created the first living organism with a genetic code containing two artificial DNA bases in addition to the standard four letters: A, C, G and T.

The letters are the four DNA subunits, or nucleotides, which link together to make strings of genetic code. In most organisms, the nucleotides pair up (A’s with T’s and C’s with G’s), forming rungs of a twisted DNA ladder.

Floyd Romesberg of the Scripps Research Institute in La Jolla, Calif., and colleagues expanded the genetic language using a pair of synthetic nucleotides, called d5SICS and dNaM, which together make up a new rung of DNA.

The researchers engineered DNA in Escherichia coli bacteriato contain the unnatural bases and process them like normal DNA. They also equipped the bacteria with molecular machinery from algae to import fresh supplies of the foreign nucleotides for DNA copying and repair. Their results appear May 7 in Nature.

The authors say that the extended genetic vocabulary has broad potential, including for creating DNA-based nanomaterials and proteins with exotic abilities.